U.S. patent specification No. 3,168,553 discloses a process in which olefins are carbonylated in the presence of a catalyst complex comprising a trialkylphosphine together with cobalt, ruthenium, rhodium or iridium. However, this process requires the use of high pressures, and its selectivity towards the desired product is often unsatisfactory. For instance, the carbonylation of ethylene in the presence of ethanol and a CO.sub.2 (CO).sub.8 catalyst leads to the formation not only of ethyl propionate, but also of large quantities of by-products, such as diethyl ketone and acetaldehyde.